1. FIELD OF INVENTION: The present invention relates to acoustic well logging.
2. DESCRIPTION OF PRIOR ART: In acoustic well logging, acoustic waves are emitted from a wave transmitter or source in a well borehole into subsurface formations to obtain information about the formations. This is done by detecting the acoustic waves after their travel through the formations to a receiver/transducer spaced from the source. The manner in which the transmitted waves are modified during travel through the formation gives indications of the nature of the formation. One area of interest is detection of fractures in the formations. However, polarized acoustic shear waves or S-waves were necessary for this purpose.
It has been known to use piezoelectric crystals or magnetostrictive materials to form pressure waves in borehole fluid to form acoustic signals. A portion of the pressure wave energy was then coupled into the earth formations surrounding the borehole. Such a technique was often effective in an open or uncased borehole. However, in a cased borehole almost all of the signal energy remained trapped in the casing.